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Full text of "Handbook Of Chemical Engineering - I"

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prism A, one field increases and the other field decreases in intensity A setting is obtained when the two fields match.
In order to determine the proper brightness at which to operate the electric lamp illuminating the slit S2) the pyrometer is sighted on a source of standard brightness. This consists of an amyl-acetate lamp with a flame gage having a window of ground glass. The flame is adjusted to a specified height. The ground glass window illuminates the slit Si. The analyzer nicol A is set at a specified normal point or angle marked on the instrument. The current through the electric lamp is then varied by means of a rheostat until the two fields are matched, and the current is read from the ammeter. This process should be repeated several times and a mean value of the current settings obtained. In using this instrument the current is adjusted to this mean value. The electric lamp burns at a high temperature and consequently deteriorates noticeably. Hence the above adjustment on the normal point requires frequent redetermination. For high precision the adjustment should be made both before and after a series of temperature readings. In the industrial plant once a day or once a week is sufficient depending upon the amount of use.
The calibration of the instrument follows the law
log tan <p =* a -f b/#
where <? is the angular reading of the analyzer, # the absolute temperature, and a and b empirical constants. The relation between log tan v and 1/tf is linear. Two calibration points serve to determine a and b and a table or plot may be made ofvv stC. ( = $  273). Usually such a table is furnished with the pyrometer, or the instrument may be sent to the Bureau of Standards for calibration.
The instrument described above is satisfactory for temperatures greater than 900C. In the temperature range 700 to 900C. the intensity of light from the furnace sighted upon is insufficient to permit accurate settings. Hence for temperatures from 700 up the direct-vision
spectroscope P is replaced by  a                                TABLE 10
red-glass screen, or the objective lens Oi is made of red glass, and the slits Si and $2 are of much wider opening.
On account of stray light the Wanner pyrometer is not accurate at very small or very large angular readings. Moreover the temperature increases so fast at large angles that the angles would have to be observed with extreme precision in order to obtain any accuracy when expressed in degrees of temperature. The range of the instrument is thus confined to from about 10 to 80 angular degrees. The following table illustrates the relation between angular readings arid temperature for a particular instrument. Different instruments may have entirely different or practically identical calibrations as desired depending upon the adjustment of the normal point.
Thus the above instrument is satisfactory for temperatures up to 1,500C. For higher temperatures an absorption glass is mounted in front of the slit Si which decreases the light from the furnace in a known ratio. The third column shows the
	Temperature, degrees Centigrade	
Angle, degrees	Without	With
	absorption	absorption
	glass	glass
10	900	1,390
20	990	1,575
30	1,060	1,725
40	1,120	1,865
50	1,185	2,020
60	1,255	2,200
70	1,360	2,460
80	1,535	3,030